The present invention relates to a centrifugal wafer drying apparatus used for drying after washing a semiconductor wafer such as a silicon wafer.
FIGS. 1A and 1B show a prior art centrifugal wafer drying apparatus having carriers 2 each receiving silicon wafers in a stack. Each of the carriers 2 has grooves 3 for accomodating wafers 1. Each of the grooves 3 extends along substantially half a periphery of a wafer 1, and has clearance with respect to the thickness of the wafer, e.g., it has a thickness of two to three times the thickness of the wafer to facilitate loading (insertion) and unloading (removal) of the wafer. The apparatus further comprises a cylindrical container 4, and cradles 5 made of stainless-steel disposed in the container 4 to hold the carriers 2. The cradles 5 are mounted on a rotor 6, which is fixed to a rotor shaft 7. The rotor shaft 7 is driven by a motor 8, so that the rotor 6 rotates together with the cradles mounted thereon. Water remaining on the wafers disperse due to the centrifugal force and the wafers 1 are thereby dried.
FIG. 2A and FIG. 2B are a plan view and a partial side view of the rotor 6. The rotor 6, disposed in the container 4 has an opening 6a, through which clean air, which has passed a filter (not shown) provided in a lid 4a of the container 4, is supplied into the rotor 6 and air streams 11 (FIG. 3) are created along the surfaces of the wafers 1, being limited by walls 6b acting as guides. The air streams also serve to disperse water from the surfaces of the wafers 1.
Each of the cradles 5 is pivotably mounted on a pivot pin 5a extending at right angles with the rotor shaft 7 and with the direction of a radius from the axis of rotation RX which is an extension of the axis of the rotor shaft 7. Each of the cradles 5 is pivotable between the positions A and B, about 90.degree., as indicated by an arrow C. At the position A, the rotor is at rest and carriers 2 are loaded in or unloaded from the cradle 5. For this purpose, the cradles 5 each have an opening 5b which faces upward for loading and unloading of the carrier. Each of the carriers 2 has an opening 2a for loading and unloading of wafers in and out of the grooves 3. The opening 2a of the carrier 2 also faces upward when the cradle is at the position A. The cradle 5 is at the position B while the rotor 6 is rotating.
When the cradle 5 is at the position B, the opening 2a of the carrier 2 faces the axis of rotation RX of the rotor. At the position B, half the periphery of each wafer that is covered by the groove is positioned on the radially outer side with respect to the axis RX of rotation of the rotor 6. At the position B, the cradle 5 is engaged with a cradle stopper 9. The stopper 9 is provided to hold the cradle 5 in such a position that the surfaces of the wafers 1 are substantially orthogonal to the axis of rotation of the rotor 6.
Examples of prior art references showing this type of drying apparatus are Japanese Patent Application Laid-open No. 36930/1984, Japanese Patent Application Laid-open No. 160130/1982 and Japanese Patent Application Laid-open No. 8823/1981.
The above-described apparatus however has a disadvantage. This is explained with reference to FIG. 3, which is a section view taken along the periphery of wafers as received by the grooves. When the rotor 6 is at rest, the wafers 1 are at positions D where the lower surfaces 1a of the wafers 1 abut upwardly-facing edges 3a of the grooves 3. This is due to gravity. When the rotor 6 is rotating, the centrifugal force acts in the direction F, substantially parallel with the surfaces of the wafers 1, so that the wafers 1 vibrate between the position D, and the position E where the upper surfaces 1b abut the downwardly-facing edges 3b of the grooves 3. The actual condition which the individual wafers assume is affected by the air stream or wind caused by the rotation of the rotor. Thus, the resultant vibration of the wafers generates dust from the parts of the wafers 1 and the grooves 3 where they collide with each other. The vibration of the wafer also causes the air stream 11 to be unstable, and the effect of the water removal by the air stream 11 is reduced. The result is an unsatisfactory drying.